fix(tracing): Add back support for formatting derived scalar types.

This is a partial revert of fix(tracing): Fix formatting of `url.URL` attri... (!236 - merged). While using a type switch is more efficient, it also differentiates between a string type and a derived type, such as ext.SpanTypeEnum. By reverting back to using reflect for the type match, we fix this regression.

This change keeps the fmt.Stringer interface match, which is used for *url.URL.

A test case for ext.SpanTypeEnum has been added.